Chalk, Flint and Wealden Clay

Drive twenty minutes north from Seaford and the ground under your wheels changes completely, and so does what happens to paving laid on it.

I work both sides of that line and the difference shows up in the work every week.

The chalk

The South Downs are chalk: a soft, white, very permeable limestone laid down as marine sediment. It runs under Seaford, Newhaven, Peacehaven, Lewes, Alfriston and the western half of Eastbourne.

It drains, fast. Rain goes straight through rather than sitting in the sub-base. That is the single most useful property chalk has for anybody with a driveway.

It does not shrink or swell. Chalk is dimensionally stable through the seasons, so paving laid on it does not move with the weather.

It is soft but stable under load. Properly compacted it makes a decent sub-base.

The practical result is that I very rarely get called to a settled or moving drive on the chalk. When paving fails here it is almost always a surface problem, and surface problems are the ones that can be cleaned or repaired.

The complication chalk brings is run-off staining. Chalk-laden water running off a bank or a border onto paving leaves a pale, hard deposit that people mistake for efflorescence. It is calcium carbonate, and unlike efflorescence it keeps arriving as long as the source does. Fixing the drainage matters more than cleaning it.

The flint

Flint occurs as nodules within the chalk, and it is the reason so many Sussex walls look the way they do. It is almost pure silica, extremely hard, and completely non-porous.

For cleaning purposes flint itself is indestructible. What holds a flint wall together is lime mortar, and that is among the easiest things to destroy with a pressure washer.

Pressure blows lime pointing out from between the flints in seconds, and once that has started, water gets into the core of the wall. Flint walls have a rubble core that must stay dry.

So the rule on flint is that the jet does not go near it. If a flint wall genuinely needs cleaning, it is a soft wash with a gentle rinse, and quite often the correct advice is to leave it.

The clay

North of the downs the geology changes to the Weald: a mix of clays and soft sandstones. Ringmer, Uckfield, Heathfield, Hailsham and Battle are all on it to varying degrees.

Weald clay behaves in three ways that matter.

It does not drain. Water sits in it. A sub-base on clay stays wet for weeks after rain, and it keeps feeding moisture up into the paving above. That is why drying times before re-sanding are so much longer inland, and why a winter job there is nearly always two visits.

It shrinks and swells. Clay minerals take water into their structure and expand, then contract as they dry. The movement across a dry summer and a wet winter is enough to lift and drop paving that was laid without a proper compacted sub-base.

That is the cause of the seasonal rocking I see on inland patios: slabs that move in February and sit flat in August. It is the ground breathing, not the paving failing, and no amount of cleaning changes it.

It stays cold. Wet clay warms slowly, which keeps surface temperatures down and extends the damp period further.

What that means town by town

On the coastal chalk, at Seaford, Newhaven and Peacehaven, the enemy is salt and exposure. Paving is stable underfoot and the problems are on the surface.

At Lewes and Alfriston, on the chalk but in valleys, drainage is good but shade and damp air are the issue.

At Polegate and Hailsham, on the transition to lower, heavier ground, the water table is the controlling factor and drying times are long.

At Ringmer, Uckfield and Heathfield, on the clay proper, seasonal movement, tannin staining from tree cover and very long drying times all apply together.

At Battle, on Wealden sandstone, the stone itself is porous enough that the growth gets inside it, which is a different problem again.

The Ashdown sandstone

Worth its own paragraph because it catches people out.

The High Weald around Battle and Heathfield sits on the Ashdown Formation, and where local stone has been used for paving, it is considerably more porous than the imported Indian sandstone most modern patios use.

Algae colonises the stone rather than sitting on it. You clean the surface, the population inside is untouched, and it regrows from within over the following months. That is why customers there report their patio greening up far faster than the general advice suggests.

It is also the one place on my patch where I recommend sealing without much hesitation, because lining the pores is the only thing that actually interrupts the cycle.

The practical takeaway

If you are on chalk near the coast: your paving is stable, your enemy is salt and algae, and the useful spend is a biocide.

If you are on clay inland: expect seasonal movement, expect long drying times, expect tannin staining under trees, and expect a two-visit job in winter.

If you are on Ashdown sandstone: the growth is in the stone, and sealing is worth it.

None of this is something you can change. It is worth knowing because it explains why your patio behaves differently from your sister’s twenty miles away, and it stops you blaming the last person who cleaned it.

How to tell which you are on

You do not need a geological map, though the British Geological Survey publishes one free online and it is worth ten minutes.

Dig a hole in a border. Chalk is white or very pale and crumbles between your fingers with visible flint nodules in it. Weald clay is grey-brown, sticky when wet, and forms a ribbon if you roll it between your palms. Ashdown sand is orange-brown and gritty.

Look at the old walls. Flint walls mean chalk underneath, because the flint came out of the ground nearby. Sandstone walls mean you are on the Weald.

Watch a puddle. On chalk, standing water after rain is gone within hours. On clay it can sit for days.

Look at what grows. Chalk downland has short springy turf, wild thyme and orchids. Weald clay grows oak and bracken.

Ask how deep the drains are. On clay, soakaways frequently do not work and properties are more likely to be on mains surface water drainage.

What each means when you are having work done

On chalk. A properly laid drive will stay level. Cleaning is straightforward, drying is quick, and re-sanding usually happens the same day. Expect algae and salt rather than movement.

On clay. Expect two visits between November and March. Expect some seasonal movement in older paving and do not assume it means the drive is failing. Expect tannin staining under trees. Budget for the biocide treatment.

On Ashdown sand. Expect faster regrowth than the general advice suggests, and treat sealing as worthwhile rather than optional.

The one thing worth doing about clay movement

You cannot change the ground. You can change how much water gets into it.

Most seasonal movement under paving is made worse by water concentrating in one place: a downpipe emptying next to the drive, a gully that has silted up, a soakaway that has stopped working, or a fall that directs the whole roof’s worth of water at one corner.

Fixing that is a drainage job rather than a paving job, and it is frequently cheap. Extending a downpipe run by two metres so it discharges into a proper gully instead of onto the edge of the drive can stop a settlement problem that somebody has been quoting hundreds to relay.

If part of your drive moves and the rest does not, go and stand outside in heavy rain and watch where the water goes. It is the most useful diagnostic available and it costs nothing but getting wet.